Zigzag Nanoribbons by Edge Effects - American Chemical Society

Letter pubs.acs.org/JPCL

Enhanced Li Adsorption and Diffusion on MoS2 Zigzag Nanoribbons by Edge Effects: A Computational Study Yafei Li,†,‡ Dihua Wu,† Zhen Zhou,†,* Carlos R. Cabrera,‡ and Zhongfang Chen‡,* †

Tianjin Key Laboratory of Metal and Molecule Based Material Chemistry, Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Computational Centre for Molecular Science, Institute of New Energy Material Chemistry, Nankai University, Tianjin 300071, China ‡ Department of Chemistry, Institute for Functional Nanomaterials, University of Puerto Rico, Rio Piedras Campus, San Juan, PR 00931 S Supporting Information *

ABSTRACT: By means of density functional theory computations, we systematically investigated the adsorption and diffusion of Li on the 2-D MoS2 nanosheets and 1-D zigzag MoS2 nanoribbons (ZMoS2NRs), in comparison with MoS2 bulk. Although the Li mobility can be significantly facilitated in MoS2 nanosheets, their decreased Li binding energies make them less attractive for cathode applications. Because of the presence of unique edge states, ZMoS2NRs have a remarkably enhanced binding interaction with Li without sacrificing the Li mobility, and thus are promising as cathode materials of Li-ion batteries with a high power density and fast charge/discharge rates.

SECTION: Energy Conversion and Storage; Energy and Charge Transport

M

significantly shortened. For example, Dominko et al.27 demonstrated that MoS2 nanotubes have highly reversible capacity (∼385 mAh/g) and excellent cycle stability. In particular, several groups28−31 found that restacked MoS2 nanoplates exhibit superior rate capability and cycling stability as anode materials for LIBs. They attributed the better performance of restacked MoS2 layers to the enlarged lattice parameter in the c direction and surface area, which facilitate the Li diffusion and intercalation. Following this argument, we can expect that MoS2 monolayer, which can be obtained by further exfoliating MoS2 nanoplates, has even better performance as LIB electrode materials. Recently, 2-D MoS2 nanosheets with one or few layers have been achieved through different methods,32−40 and their applications have been explored, such as in nanotransistors.41−44 More excitingly, shortly after the theoretical predictions,45,46 Wang et al. realized single-layered MoS2 nanoribbons (MoS2NRs) experimentally.47 Not suprisingly, the electrochemistry of MoS2 monolayerrelated materials has been attracting a lot of interest ever since their synthesis. Chang et al. synthesized a series of MoS2 sheet/ graphene composites that exhibit excellent rate capability and cycling stability as anode materials for LIBs.48−51 Recently,

olybdenum disulfide (MoS2), a typical layered inorganic material, structurally well resembles graphite. However, instead of a layered planar structure in graphite, MoS2 is constructed by triple atomic layers, in which Mo atoms in the trigonal prismatic coordination are sandwiched between two layers of pyramidal S atoms. Similar to graphite, these MoS2 layers are held together by the weak van der Waals (vdW) interactions. Such a unique structure endows MoS2 many excellent catalytic,1−3 photovoltaic,4,5 and lubricant properties,6−8 and at the same time provides sufficient interlayer spaces for intercalating foreign molecules and atoms.9−12 Nowadays lithium ion batteries (LIBs) are widely used as energy storage media for consumer electronics and are also promising for electric vehicles and electric grid applications, among others. Normally an LIB consists of anode, cathode, and electrolyte. The anode of LIBs is usually made from carbon materials, whereas the cathode is usually built out of metal oxide. Ideally, a good cathode material should have high Li intercalation voltage, whereas a good anode material should have low Li deintercalation voltage, and both cathode and anode materials prefer high Li mobility.13,14 Because of the unique layered structure, MoS2 highly favors the reversible Li+ intercalation/deintercalation and has long been considered to be an ideal electrode material for advanced LIBs.15−19 The recent breakthroughs in fabricating MoS2 nanostructures20−26 make MoS2 even more attractive for LIB applications because in nanostructures the Li diffusion path could be © XXXX American Chemical Society

Received: June 19, 2012 Accepted: July 29, 2012

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dx.doi.org/10.1021/jz300792n | J. Phys. Chem. Lett. 2012, 3, 2221−2227

The Journal of Physical Chemistry Letters

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(P63/mmc). However, the most stable form of bulk phase of MoS2 is 2H type,60 and MoS2 plate-like crystals usually grow in the 2H type, which is a stable form up to 1000 °C.61 Therefore, only the 2H type was considered in our computations. To avoid the interaction between two Li atoms safely, we used a 4 × 4 × 2 supercell for MoS2 bulk, which consists of 32 Mo atoms and 64 S atoms. Two representative sites for lithiation are available in MoS2 bulk: (1) the tetrahedral site formed by three S atoms from upper triple layer and one S atom from lower triple layer (Figure 1a) and (2) the octahedral site in which Li can bind to three S atoms from each of the triple layers (Figure 1b).

Liang et al. demonstrated experimentally that highly exfoliated graphene-like MoS2 monolayer is a good cathode material of magnesium (Mg) batteries.52 Yang et al. theoretically predicted that MoS 2 NRs could also be promising cathodes of rechargeable Mg batteries.53 This naturally raises an interesting question: could MoS2 monolayer and nanoribbons have the potential to be used as cathode materals of LIBs? To our best knowledge, this issue has not been addressed until now. In this work, we systematically investigated the adsorption and diffusion of Li atom on MoS2 bulk, bilayer, monolayer, and zigzag MoS2NRs (ZMoS2NRs) by means of density functional theory (DFT) computations and explored the potential of using MoS2 monolayer and ZMoS2NRs as cathode materials of LIBs. MoS2 bulk binds strongly with Li and has a moderate Li diffusion barrier; Reducing dimensionality to bilayer and monolayer simutaneously decreases the Li binding energy and diffusion barrier. More importantly, because of the edge effect, both the binding strength and the mobility of Li (by lowering the diffusion barrier) can be enhanced in zigzag MoS2 nanoribbons. Therefore, ZMoS2NRs are promising as cathode materials of Li-ion batteries with a high power density and fast charge/discharge rates. Computational Details. Our DFT computations were carried out by using an all-electron method within a generalized gradient approximation (GGA) for the exchange-correlation term, as implemented in the DMol3 code.54,55 The double numerical plus polarization (DNP) basis set and PW91 functional were adopted. 56 Self-consistent field (SCF) computations were performed with a convergence criterion of 10−6 a.u. on the total energy and electron density. To ensure high-quality numerical results, we chose the real-space global orbital cutoff radius as high as 4.6 Å in all computations. The Brillouin zones of MoS2 bulk, nanosheets, and nanoribbons were sampled with 4 × 4 × 4, 4 × 4 × 1, and 1 × 1 × 8 k points, respectively. The transition states were located by using the synchronous method with conjugated gradient (CG) refinements.57 This method involves linear synchronous transit (LST) maximization, followed by repeated CG minimizations, and then quadratic synchronous transit (QST) maximizations and repeated CG minimizations until a transition state is located. It is well known that standard PW91 function is incapable of giving an accurate description of weak interactions. Therefore, for MoS2 bulk and bilayer, we adopted a DFT+D (D stands for dispersion) approach with the Ortmann−Bechstedt−Schmidt (OBS) vdW correction.58 This approach is a hybrid semiempirical solution that introduces dispersion correction of the form C6R−6 in the DFT formalism. The computed lattice constant c of MoS2 bulk (12.37 Å) is in good agreement with experimental value (12.32 Å).59 We define the Li binding energy, Eb(Li), as Eb(Li) = Etot(MoS2) + Etot(Li) − Etot(MoS2−Li), where Etot(MoS2−Li), Etot(MoS2), and Etot(Li) are the total energies of Li-adsorbed MoS2 (bulk, monolayer, bilayer or nanoribbon), Li atom, and MoS2, respectively. According to this definition, a more positive binding energy indicates a more favorable exothermic lithiation reactions between MoS2 and Li. Results and Discussion. Adsorption and Dif f usion of a Single Li Atom in Bulk MoS2. First, we examined the adsorption and diffusion of a single Li atom in bulk phase of MoS2. MoS2 layers can be stacked in two patterns: one is rhombohedral type (3R) with three-molecule layers per unit cell and the other is hexagonal type (2H) with two-molecule layers per unit cell

Figure 1. Side views of geometries of a Li atom adsorbed at the tetrahedral site (a) and octahedral site (b) of MoS2 bulk. The cyan, yellow, and pink balls denote Mo, S, and Li atoms, respectively. (c) Energy profiles for Li diffusion in MoS2 bulk and bilayer from an octahedral (Oh) site to another, passing through a tetrahedral (Th) site.

Our computations show that Li atom prefers to adsorb at the octahedral site (Figure 1b) with a binding energy of 3.08 eV, and the newly formed Li−S bonds are uniformly 2.45 Å. According to the Hirshfeld population analysis, Li possesses only 0.05 |e| positive charge, indicating that there is certain covalent component in Li−S bonds. The lithium adsorption at the tetrahedral site is also energetically favorable (with a binding energy of 2.64 eV); in this case, the lengths for upper and lower Li−S bonds are 2.23 and 2.12 Å, respectively, and Li donates 0.06 |e| charge to MoS2 bulk. Then, we studied the Li diffusion in MoS2 bulk between two neighboring octahedral sites, passing through a tetrahedral site by using LST/QST method (Figure 1c). The computed diffusion barrier (0.49 eV) indicates that Li can readily diffuse in bulk MoS2. However, to enhance further the Li mobility, this diffusion barrier still needs to be decreased. It is expected that in exfoliated MoS2 nanosheets Li can diffuse faster than in MoS2 bulk. Adsorption and Dif f usion of a Single Li Atom in MoS2 Bilayer and Monolayer. To examine whether the Li mobility can be enhanced in exfoliated MoS2 nanosheets, we investigated the adsorption and diffusion of Li in both MoS2 bilayer and monolayer. In MoS2 bilayer, the octahedral site is more favorable for lithiation than the tetrahedral site (the binding energies are 2.71 and 2.52 eV, respectively). Note that even the highest Li 2222



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Although the 2D MoS2 nanosheets have much higher Li mobility, their decreased Li binding energies do not make them ideal candidates for cathode materials. Note that high Li binding energy, or high Li intercalation voltage, is required to ensure the thermodynamic stability in cathodes. Rolling MoS2 nanosheets into nanotubes may increase the Li binding energy due to the curvature effect; however, our computations revealed that the Li bonding energies in thin MoS2 nanotubes are still lower than that in MoS 2 bulk. (See the Supporting Information.) Therefore, increasing the Li binding energy without sacrificing the Li mobility is the problem for us to conquer. Adsorption and Dif f usion of a Single Li Atom in MoS2 Nanoribbons. Inspired by the theoretical finding by Barone et al.67 that cutting graphene into graphene nanoribbons (GNRs) can significantly enhance the Li binding energies due to the presence of reactive edges, we expected that the enhanced Li binding could also exist in MoS2NRs. In the following parts, we put our emphasis on the adsorption and diffusion of Li on the MoS2NRs. Similar to GNRs, two types of MoS2NRs, with zigzag or armchair edges, can be obtained by cutting a MoS2 monolayer. However, previous theoretical studies45,47 demonstrated that ZMoS2NRs are more stable than the armchair ones, and the experimentally realized MoS2NRs47 also have zigzag edges. Therefore, in this work, we only considered the Li adsorption and diffusion on the MoS2NRs with zigzag edges. The 8ZMoS2NR (refer to ref 45 for the definition of the width of MoS2NRs) was chosen as a model for the study. In particular, the Mo edge of 8-ZMoS2NR is half-passivated by S atoms, and the S edge is fully saturated with S atoms, which has been confirmed theoretically to be the energetically most favorable passivation pattern (Figure 3).47

binding energy in MoS2 bilayer is 0.37 eV lower than that for the most favorable site in MoS2 bulk (3.08 eV). MoS2 bilayer has a little larger interlayer distance than the bulk (6.37 Å vs 6.18 Å). Upon lithiation, the newly formed Li−S bonds at the octahedral site in MoS2 bilayer are also slightly longer than the corresponding ones in MoS2 bulk (2.51 Å vs. 2.45 Å). As expected, the elongated interlayer distances in MoS2 layer lead to the facilitated Li diffusion: the computed Li diffusion barrier between two neighboring octahedral sites in MoS2 bilayer (0.32 eV) is 0.17 eV lower than that of MoS2 bulk (Figure 1b). When MoS 2 bulk is eventually exfoliated into MoS 2 monolayer, there are also two representative adsorption sites for Li lithiation, namely, the hollow site (H) above the center of the hexagon (Figure 2a) and the top site (T) directly above one

Figure 2. Geometries of a Li atom adsorbed at the T site (a) and H site (b) of MoS2 monolayer. (c) Energy profiles for Li diffusion on MoS2 monolayer from a T site to another, passing through an H site.

Mo atom (Figure 2b), which are actually derived from the bulk octahedral and tetrahedral sites, respectively. In both H and T sites, Li atoms are coordinated by three S atoms. We also considered the possibility for the Li atom atop a S atom, but this configuration was excluded because Li moves to the T site spontaneously upon relaxation; similar phenomena were found when adsorbing other metal atoms on MoS2 in previous theoretical studies.62−65 The T site is energetically more favorable to bind a lithium atom than the H site (Li binding energies are 2.12 and 2.01 eV for T and H sites, respectively). For lithiation at the T site, the mean Li−S bond length is 2.37 Å, and at this case Li possesses a 0.36 |e| positive charge, which is much more pronounced than that in MoS2 bulk (0.05 |e|). For lithiation at the H site, the mean Li−S bond length is 2.43 Å, and there is about 0.32 |e| charge transfer from Li atom to the MoS2 monolayer. The diffusion of Li on MoS2 monolayer occurs by migrating from a T site to another, passing through a H site (Figure 2c). The computed energy barrier (0.21 eV) is much lower than that in MoS2 bulk (0.49 eV) and is also lower than that in MoS2 bilayer (0.32 eV). A similar result was also found in TiS2-related materials.66 The difference of diffusion barriers among MoS2 bilayer, monolayer, and bulk would have a significant effect on the mobility of Li. By using Arrhenius equation (D ∝ e(Ea/kT)), we can quantitatively estimate that Li mobility in MoS2 bilayer and monolayer can be increased by a factor of 102 and 104, respectively, compared with that in MoS2 bulk at room temperature.

Figure 3. (a) Top view of geometry of 8-ZMoS2NR. All examined Li adsorption sites are denoted with red characters. (b,c) Side views of S and Mo edges, respectively.

Because of the unsymmetrical structural property, many more unique sites for lithiation are available along the transverse direction of ZMoS2NRs. In this study, we considered all possible sites throughout the ribbon for Li adsorption on 8ZMoS2NR, including 8 T sites and 8 H sites on the basal plane and two sites on the side plane (S1,S2). Note that S1 and S3 are two-fold hollow sites, whereas S2 is a four-fold hollow site. The computed Li binding energies, mean Li−S bond lengths, and the Hirshfeld charges of Li for all the examined sites for 82223



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Table 1. Computed Li Binding Energies (Eb), Mean Li−S Bond Lengths (LLi−S), and Hirshfeld Charges of Li (QLi) for All the Examined Li Adsorption Sites of 8-ZMoS2NR site

T1/H1

T2/H2

T3/H3

T4/H4

T5/H5

T6/H6

T7/H7

S1/S2

S3

Eb LLi−S QLi

3.59/3.40 2.39/2.43 0.31/0.33

3.10/2.92 2.40/2.43 0.33/0.35

2.94/2.79 2.40/2.43 0.33/0.36

2.85/2.73 2.40/2.43 0.33/0.36

2.84/2.74 2.40/2.43 0.33/0.36

2.89/2.80 2.39/2.42 0.33/0.36

2.96/2.91 2.40/2.43 0.33/0.35

3.62/3.47 2.33/2.71 0.34/0.36

3.26 2.32 0.40

ZMoS2NR into metallic (Figure 4b). These metallic states would enhance the interaction between Li and 8-ZMoS2NR and directly lead to the stronger binding among them. Moreover, the metallic states at the zone of Fermi level are mainly contributed by the edge atoms, especially those at the S edge (Figure 4c), which can explain why the edge sites have more pronounced binding energies than the inner sites and the S edge is more favorable for Li binding than the Mo edge. Similar edge states can also be found in zigzag GNRs, which are believed to be responsible for the enhanced Li binding strength at the edge sites.67 Note that GNRs have been experimentally used as advanced anode materials,68 whereas ZMoS2NRs are a good candidate for cathode applications. To examine the edge effect on the Li mobility, we further computed the Li diffusion barrier along different pathways, namely, along the periodic direction (axis direction, Figure 5) and along the transverse direction (Figure 6) of 8-ZMoS2NR.

ZMoS2NR are summarized in Table 1. The T8 site does not correspond to a local minimum; upon relaxation, the Li atom at T8 site moves to the edge and binds with two terminal S atoms (S3 site). Among all the sites of 8-ZMoS2NR considered here, the twofold hollow S1 site at the edge has the highest Li binding energy (3.62 eV), followed by two other edge sites, namely, the tetrahedral T1 site and four-fold hollow S2 site (3.59 and 3.47 eV, respectively). On the basal plan, the Ti sites are generally energetically more favorable than the corresponding Hi sites, which is similar to the case of MoS2 monolayer. All these examined adsorption sites have a higher Li binding energy than that of MoS2 monolayer, and some sites even have a higher Li binding energy than that of MoS2 bulk, and thus cutting MoS2 monolayer into 1D ZMoS2NRs can effectively enhance the Li binding energy, which is similar to the case of GNRs.67 Moreover, the sites close to the edges have higher Li binding energies than the inner sites, implying a remarkable edge effect. In particular, the S edge of the ZMoS2NRs is more favorable to bind Li than the Mo edge. To get a deeper understanding of the mechanism behind the enhanced interaction strength of Li on 8-ZMoS2NR, we computed the density of states (DOS) of 8-ZMoS2NR and compared with that of MoS2 monolayer. Consistent with previous studies,45,46 2D MoS2 monolayer is semiconducting with a 1.62 eV band gap (Figure 4a). The conduction band minimum (CBM) and valence band maximum (VBM) are both contributed by the 3d states of Mo atoms throughout the layer (not shown here). In other words, there is no particular reactive site in MoS2 monolayer. In sharp contrast, when MoS2 monolayer is cut into 1D zMoS2NRs, several substantial peaks appear at the zone of Fermi level, which tune 8-

Figure 5. Top (upper) and side (bottom) views of Li diffusion paths on 8-ZMoS2NR along the axis direction. The numbers indicate the diffusion barriers in electronvolts. The double-headed arrows denote that Li atoms can diffuse in two directions.

Along the periodic direction of 8-ZMoS2NR, seven diffusion paths (Pi = Ti → Hi → Ti, i = 1−7) on the basal plan and two diffusion paths (P8 = S2 → S1 → S2, P9 = S3 → S3) on the side plan can be identified (Figure 5). Similar to the case of binding energies, the Li diffusion barriers are also very sensitive to whether the binding sites are close to the edge or not. On the basal plan, the Li diffusion barrier decreases gradually from the S edge to the Mo edge, and some diffusion paths even have a lower energy barrier than that of MoS2 monolayer. The lowest (0.14 eV) and the highest (0.32 eV) diffusion barriers both come from the diffusion paths of side plane. On the side plane of S edge, a Li atom only needs to conquer an energy barrier of 0.14 eV to migrate from a S1 site to another, passing through a S2 site. However, the pathways along the transverse direction on the basal plane of 8-ZMoS2NR (P10 = T5··· → ···T1, P11 = T5 → T6 → T7, see Figure 6) have lower activation barriers than the axis direction considered above; thus in reality, the Li atoms will prefer the migration along the transverse direction. Regardless where the Li atom is adsorbed on the nanoribbon, it will migrate to the edge by crossing the activation barriers, which decrease gradually toward the edge. If the Li atom is first adsorbed at the site near the S edge, then it will prefer to diffuse toward the S edge, otherwise toward the Mo edge (Figure

Figure 4. Density of states (DOS) of MoS2 monolayer (a) and 8ZMoS2NR (b). Electronic profile of metallic states at the Fermi level of 8-ZMoS2NR (c). 2224



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Z.Z.); zhongfangchen@ gmail.com (Z.C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support in China by NSFC (21073096), MOE Innovation Team (IRT0927), 111 Project (B12015), and the Fundamental Research Funds for the Central Universities and in the USA by NSF Grant EPS-1010094 and Department of Defense (Grant W911NF-12-1-0083) is gratefully acknowledged. The computations were partially performed on TianHe-1(A) at National Supercomputer Center in Tianjin, China.

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Figure 6. (a) Schematic representation of two diffusion paths (P10,P11) on the basal plane of 8-ZMoS2NR along the transverse direction. (b,c) Energy profiles for P10 and P11, respectively.

6b,c). This edge-preferred Li diffusion, which was also found in GNRs,69 may be explained by the unique edge effects. In comparison, in the nanostructures without strong edge effects, such as Si nanowires, Li prefers to diffusion from the surface to the center rather than the opposite direction.70−72 Overall, the above results imply that once used as LIB cathode materials, ZMoS2NRs can present fast charge/discharge capability. Conclusions. To summarize, we performed DFT computations to disclose the Li adsorption and diffusion in MoS2 bulk, bilayer, monolayer, and ZMoS2NRs. Comprehensive computations demonstrated that the Li mobility can be enhanced by the dimensionality reduction. However, the Li binding energy of MoS2 nanosheets is lower than that of MoS2 bulk, which makes MoS2 nanosheets not ideal candidate for cathode applications. Cutting MoS2 nanosheets into ZMoS2NRs can significantly enhance the Li binding energy and at the same time preserve the high Li mobility. Therefore, ZMoS2NRs, which have been recently realized experimentally, are promising as cathode materials of Li-ion batteries with high power densities and fast charge/discharge rates. Our theoretical studies will hopefully inspire experimental studies on ZMoS2NRs as cathode materials.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Variation of the binding energy of Li on the outside surface T site of (n,n) (6 ≤ n ≤ 10) and (n,0) (9 ≤ n ≤ 14) MoS2 nanotubes as a function of tube diameter. This material is available free of charge via the Internet at http://pubs.acs.org. 2225



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Zigzag Nanoribbons by Edge Effects - American Chemical Society

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